I have used thick deck kits for other Delta widespread faucets. Check on the kit for a Roman tub, it may have the same body as your faucet.

: I have a great old pedestal sink, ca 1900, that seems to be made out of soapstone (?) with a porcelain glaze. It was fitted out with separate hot and cold faucet taps on an odd spacing, about 13" o.c., and some weird drain control in the center. I bought a widespread (6 - 16") three-hole lav set which has valve bodies requiring a 1 1/8" hole.

: When I took the old faucets out, much to my surprise I found 1" square holes in the deck (to accomodate the square faucet body -- more weirdness). I was able to drill out the 1" square holes to 1 1/4" round with a tile bit; this stuff is actually pretty soft.

: Now my problem is that the deck is 1 3/4" thick and the valves only accommodate up to 1 1/4" deck thickness. There is no "Thick deck" kit for this (Delta Victorian) faucet set, and indeed those only seem to be available for kitchen fixtures. Hence, I want to recess the underside about 3/4" so that I can fasten the valve. If this were wood, I would put a plug in to center the drill, and use a forstner bit to create the flat-bottom seat for the nut. What sort of tool should I use on this stuff?